Plasminogen activator (PA) inhibitor Type 2 (PAI-2) is one of four immunologically distinct groups of PA inhibitors. PA inhibitors are members of the serpin gene family.
PAI-2, also termed minactivin, has been purified from the human monocytic cell line U937 (International Patent Application PCT/AU85/00191 published as W086/01212 and PCT/AU87/00068 published as W087/05628). It has also been detected in pregnancy plasma and in the conditioned medium of several human cells including peripheral blood monocutes, HT1080 fibrosarcoma cells and HEp3 laryngeal carcinoma cells.
Distinct molecular weight species of PAI-2 have been identified:
A 46 kD form is non-glycosylated and primarily cell associated. This form has been found in lysates of U937 cells and purified from the conditioned medium of phorbol ester (PMA) stimulated U937 cells (International Patent Application No. PCT/AU87/00068).
A 60 kD glycosylated form has been found to be secreted by U937 cells (Genton et al., 1987) as well as being present in the maternal plasma of pregnant women (Lecander and Astedt, 1986).
The non-glycosylated 46kD intracellular form accounts for 80-90% of PAI-2 synthesized in U937 (Genton et al., 1987) and HT1080 cells (Medcalf et al., 1988)
Examination of the primary amino acid sequence of PAI-2 reveals that the molecule lacks the characteristic transient signal peptide usually found at the amino (NH.sub.2)-terminus of secreted proteins. Although the NH.sub.2 - terminus of the purified 46 kD form of PAI-2 from U937 cells has not been determined, apparently because it is blocked (Kruithof et al, 1986), the NH.sub.2 - terminus of the 60 kD glycosylated version has been reported to be the initiator methionine (residue 1, Ye et al., 1988). This implies that secretion occurs without cleavage of a signal peptide from PAI-2. The mode of translocation of PAI-2 through the cell is not understood, however an internal signal has been proposed (Ye et al., 1988).
As is the case with most potent biologically active proteins, PAI-2 is produced in very small amounts in vivo and as such is difficult to purify and characterise by conventional biochemical approaches. The recent cloning of the gene for PAI-2 and its expression in bacterial cells (International Patent Application No. PCT/AU87/00068) now allows the production of significant quantities of purifed 46 kD PAI-2 which is needed to evaluate its biological efficacy in clinical applications. However this bacterial material is not glycosylated, nor modified post-translationally in a manner analogous to that secreted by human cells. Therefore it is desirable to produce glycosylated forms of PAI-2 using transfected mammalian cells, since the two forms of PAI-2 may differ in their biological activities e.g. binding affinity for urokinase, PA specificity, immunogenicity, in vivo half-life etc.
The native PAI-2 gene has previously been expressed in a number of heterologous mammalian expression systems (International Patent Application No. PCT/AU87/00068). Although PAI-2 is synthesized in these systems, expression levels are low, and the majority of the product (approx. 90%) is non-glycosylated and intracellular. PAI-2 produced in this form is a suitable molecule for prophylactic, therapeutic and/or diagnostic uses but its use is limited by the quantities obtainable and limited glycosylation. It is still desirable to be able to attain efficient secretion, of the 60 kD glycosylated molecule, in order to be able to use the molecule for prophylactic, therapeutic and/or diagnostic purposes on a clinical scale.
In addition to expression of PAI-2 in mammalian cells (International Patent Application No. PCT/AU87/00068), workers have attempted to secrete the 60 kD form of PAI-2. A Swedish group has attempted to secrete PAI-2 from transfected CHO cells but could not detect any secretion of 60 kD PAI-2 (Ny et al, 1988, Ny et al, 1989). A Swiss group attempted the secretion of PAI-2 using transfected WISH cells but likewise found that most of the PAI-2 material was of the intracellular form. Although some high molecular weight material was found in the culture medium, it was not demonstrated that the material was glycosylated (Belin, D., Wohlwend, A., and Vassalli, J. D. (1989); Belin, D., Wohlwend, A., Schleuning, W-D, Kruithof, E. K. O. and Vassalli, J. D. (1989)). The material may have been partially glycosylated because the mobility was different from U937 high molecular weight PAI-2.
Previous studies have shown that the addition of a heterologous hydrophobic signal domain to the NH.sub.2 - terminus of a cytoplasmic protein may result in certain circumstances in the translocation of the cytoplasmic protein across the endoplasmic reticulum (Hiebert and Lamb, 1988). Translocation across the ER is the first step along the secretory pathway. The nature of the heterologous signal peptide (e.g. whether or not it is cleaved from the protein) and the inherent properties of the cytoplasmic protein (e.g. the presence of other transport or membrane retention signals) then determine whether the chimeric protein is secreted, anchored to a cellular membrane or degraded.
Because of the complexity of the secretion mechanisms it is not possible to convert in a predictable manner a cytoplasmic non-glycosylated molecule into a secreted, glycosylated molecule. Even if by the addition of a signal sequence a cytoplasmic molecule is secreted, the processing (i.e. cleavage) of the added signal sequence may occur at different unpredictable locations.
Transient NH.sub.2 - terminal signal sequences found on most secreted proteins are identified by three distinct regions--a basic N-terminal region which may contain charged residues, a central hydrophobic core and a C-terminal region containing the signal peptidase recognition site.
There is no consensus sequence for signal peptidase recognition although von Heijne (1986) has developed rules that take account of many of the known cleavage sites. The application of these rules, however, does not necessarily allow the prediction of cleavage sites when they are applied to cytoplasmic molecules bearing heterologous signal sequences.